1. Field of the Application
The present application relates to leakage vortexes at the rotor blade tips of a turbomachine. More specifically, the present application relates to a casing designed to limit the effect of blade-tip vortexes that limit the stability of an axial turbomachine compressor. The present application also relates to a compressor and an aircraft turbojet.
2. Description of Related Art
An axial turbomachine compressor has alternating rows of rotor blades and stator vanes. The rotation of the rotor and of the blades of same helps to progressively compress the primary flow passing through the turbomachine. However, this compression involves leaks between the rotor blade tips and the surrounding casing. Indeed, mechanical clearance is required at this interface to prevent contact.
During rotation of the rotor, the blade tips sweep the internal surface of the casing and the leaks bypass the blade tips forming vortexes towards the blade lower surfaces. Each vortex creates a blocking zone against the related blade where movement of the fluid is low. In some circumstances, when the speed of the main flow is reduced, the vortex can reach the leading edge of the following blade. This can cause the flow in the blocking zone to be inverted, which may in turn make the compressor unstable. Surge phenomena may occur, which can be prevented using a casing treatment.
Document US2011/0299979 A1 discloses a turbomachine with a compressor. The compressor has a fixed stator and a moveable wheel bearing the annular rows of blades. The stator comprises an outer casing surrounding the rotor blades, said casing having annular grooves corresponding to the blades. These grooves are of variable depth to maintain the stall margin of the compressor. However, the depth and width of each groove increase the blade tip leakages, thereby limiting the compression ratio of the compressor. Moreover, the performance of the turbomachine is reduced.
Although great strides have been made in the area of casings for axial turbomachine compressors, many shortcomings remain.